MATLAB Code for designing of IIR Butterworth LPF using IIT technique

 IIR Butterworth LPF using IIT technique:
clear all;
clc;

ap = input('Gain at Passband Edge Frequency : ');
as = input('Gain at StopBand Edge Frequency : ');
wp = input('PassBand Edge Frequency(Digital): ');
ws = input('StopBand Edge Frequency(Digital): ');
pef_d = wp*pi;
sef_d = ws*pi;
ts = input('Sampling Time in sec :');
del2 = -20*log10(ap)
del1 = -20*log10(as)
pef_a = pef_d/ts
sef_a = sef_d/ts
[N,wc] = buttord(pef_a,sef_a,del2,del1,'s')

[bn,an] = butter(N,1,'s');
display('Normalized Transfer Function is, ')
Hnorm  = tf(bn,an)

[b,a] = butter(N,wc,'s');
display('Unnormalized Transfer Function is, ')
Hunnorm = tf(b,a)

[bz,az] = impinvar(b,a,1/ts);
display('Digital Transfer Function is, ')
Hz = tf(bz,az,ts)

w = 0:pi/16:pi;
display('Frequency Response is ,')
Hw = freqz(bz,az,w)
display('Magnitude Response is,')
Hw_mag = abs(Hw)
display('Phase Response is,')
Hw_phase = angle(Hw)
%subplot(2,1,1);
figure
plot(w/pi,Hw_mag,'k');
grid;
title('Magnitude Response for Butterworth Low Pass Filter ');
xlabel('Normalized Frequency, \omega/\pi');
ylabel('Magnitude |H(\omega)|');
%subplot(2,1,2);
figure
plot(w/pi,Hw_phase,'k');
grid;
title('Phase Response for Butterworth Low Pass Filter ');
xlabel('Normalized Frequency, \omega/\pi');
ylabel('Magnitude |H(\omega)|');

Use the following inputs:
Gain at Passband Edge Frequency : 0.8
Gain at StopBand Edge Frequency : 0.2
PassBand Edge Frequency(Digital): 0.2
StopBand Edge Frequency(Digital): 0.32

Sampling Time in sec :0.05

Magnitude Response:

Phase Response:

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